The invention relates to sonic waveguide delay lines, particularly sonic waveguide delay lines used for displacement measurement.
It is well known that torsional twisting may be induced on a current carrying ferromagnetic rod through interaction of a magnetic field generated by a current in the rod and a second magnetic field. The second magnetic field may be supplied by an annular magnetic or a set of discrete magnets positioned around or near the rod. Discrete magnets are preferably positioned so that their poles are perpendicular to the rod. If an annular magnet is used, it is disposed around the rod. By fixing ends of the rod and drawing the rod taut, the rod will also support the transmission of sound. Thus a rod can also act as a sonic waveguide.
Sound waves are induced in sonic wave guides by transmitting short duration electrical current pulses on the sonic waveguide. Torsional twisting is imposed on the conductor in the area where the magnetic fields produced by the current pulses interact with the magnetic field of the permanent magnets. The torsional twists propagate outwardly along the waveguide from the area of interaction as sonic pulses.
A transducer is attached to the sonic waveguide at a reference point along the rod to reconvert the sonic pulses to electrical signals. The time delay between transmission of an electrical current pulse and reception of the resulting sonic pulse is related to the displacement of the second magnetic field from the reference point. By knowing the propagation velocity of sound in the waveguide, the displacement of the second waveguide magnetic field from the transducer may be calculated.
Typical prior art sonic waveguides included a thin electrically conductive wire centered within a thin walled ferromagnetic tube. In such waveguides, the electrical pulse is transmitted down the centered wire and torsional sonic pulses are transmitted along the tube. Transducer apparatus is located at the waveguide reference point, formed from a pair of flat tapes fixed on diametrically opposite sides of the tube. A torsional sonic pulse on the sonic waveguide becomes longitudinal pulses in the tapes. Longitudinal pulse trains in the opposed tapes result in a change in the magnetization or permeability of the tapes. In the presence of a magnetic bias field a change in the magnetic field through coils wound on the tape results. This produces an induced voltage across the coils. The coils are interconnected to provide a summed signal of suitable amplitude to indicate the arrival of the sonic torsional pulse. (A permanent magnet provides the magnetic bias for the coils.)
Sonic delay lines comprising hollow tube waveguides have been used for the determination of the volume of a liquid in a tank of fixed size. The waveguides are positioned vertically in the tank, generally extending from near the bottom of the tank to above the maximum permitted depth of the stored liquid. A float is provided which rides at the top of the liquid in the tank and around or alongside of the hollow tube sonic waveguide. The float carries an annular magnet or a plurality of discrete magnets, which provide a static magnetic field with which magnetic fields generated along the internal conductor interact. As liquid depth varies, the float moves along the sonic waveguide, changing the position of the magnet with respect to the waveguide. An electrical pulse sent down a wire centered in the waveguide, having a magnetic field associated therewith, will, upon reaching the magnet, interact with the magnetic field of the magnet to generate a sonic pulse on the waveguide at about the level of liquid in the tank. The sonic pulse will be conveyed back to operating circuitry associated with the waveguide at a reference position on the waveguide, typically at or near the top of the tank.
Fuel tank depths can exceed 80 feet. This requires a sonic waveguide of at least this length to provide depth measurement. The use of rigid hollow tubes is difficult. On the one hand, the handling of rigid tubes of such length during installation of the waveguide is clumsy. Assembly of some prior art waveguides has required in situ stringing of the centered conductor wire in the tube and of its return wire back through the supports. This procedure has proven difficult and time consuming.